1. Field of the Invention
The invention relates to a liquid crystal display device, and more particularly, to a backlight unit for a liquid crystal display device.
2. Discussion of the Related Art
Until recently, display devices have typically used cathode-ray tubes (CRTs). Presently, many efforts and studies are being made to develop various types of flat panel displays, such as liquid crystal display (LCD) devices, plasma display panels (PDPs), field emission displays, and electro-luminescence displays (ELDs), as a substitute for CRTs. Of these flat panel displays, LCD devices have many advantages, such as high resolution, light weight, thin profile, compact size, and low voltage power supply requirements.
In general, an LCD device includes two substrates that are spaced apart and face each other with a liquid crystal material interposed between the two substrates. The two substrates include electrodes that face each other such that a voltage applied between the electrodes induces an electric field across the liquid crystal material. The induced electric field changes the alignment of the liquid crystal in the direction of, and according to the intensity of the induced electric field, thereby changing the light transmissivity of the LCD device. Thus, the LCD device displays images by varying the intensity of the induced electric field.
The LCD devices need a backlight unit to supply light. The backlight unit is categorized into an edge type and a direct type. The edge type backlight unit includes a light source located at a side of the LCD device. The direct type backlight unit includes a light source located at the bottom of the LCD device.
FIG. 1 is a perspective view of an edge type backlight unit according to the related art. Referring to FIG. 1, the edge type backlight unit 20 includes a light guide plate 30, a lamp 24 as a light source, a reflector 22 below the light guide plate 30, and a plurality of optical sheets 29 on the light guide plate 30. The optical sheets 29 include a diffusion sheet and at least one condensing sheet. A pattern at a bottom surface of the light guide plate 30 provides a uniform plane of light.
Since the light guide plate 30 and the reflector 22 are adjacent to each other, the light guide plate 30 may stick tightly to the reflector 22 due to static electricity produced at opposing surfaces of the light guide plate 30 and the reflector 22. The backlight unit 20 produces heat during operation, and the heat expands the light guide plate 30 and the reflector 22. However, the light guide plate 30 and the reflector 22 have different thermal expansion rates, and the reflector 22 is thinner than the light guide plate 30. Accordingly, the heat expansion may cause the reflector 22 to wrinkle. A luminance in a display region of the LCD device corresponding to the wrinkled region of the reflector 22 may be different from the luminance in another display region. Accordingly, such variations in luminance across the LCD device may cause defects, such as stains, on the display screen of the LCD device, and display quality is thus degraded.